1. Field of the Invention
The present invention relates to a hydraulic shock absorber for use in a vehicle.
2. Description of the Related Art
Japanese Patent Publication No. 42-17787, discloses a hydraulic shock absorber which is constructed with the aim of providing both operating stability and riding comfort to generate a normal damping force in response to a low frequency vibration applied to a car and a little or no damping force in response to a high frequency vibration.
As shown in FIG. 1, in such a hydraulic shock absorber, an upper pressure chamber 4 and a lower pressure chamber 5 are arranged on the upper and lower sides of a valve 3 which closes an oil passageway 2 formed in a piston 1. A chamber 6 partitioned on the upper side of the piston 1 communicates with the lower pressure chamber 5 via an oil chamber 7 whose volume varies with the internal pressure. An orifice 8 is arranged between the chamber 0 and the oil chamber 7.
According to the above construction of the hydraulic shock absorber, when frequency of a pressure change in the chamber 6 on the upper side of the piston 1 is low, a large amount of oil flows through the orifice 8 and the oil chamber 7 to the lower pressure chamber 5 to increase the oil pressure in the lower pressure chamber 5. This results in augmenting a force acting in a direction so as to close the valve 3, thereby generating a normal damping force.
On the contrary, when frequency of the pressure change in the chamber 6 is high, the orifice 8 restricts the oil flow to the oil chamber 7, thus limiting the pressure increase in the oil chamber 7. Since the pressure in the lower pressure chamber S becomes lower than that in the upper pressure chamber 4, the degree to which the valve 3 opens increases, whereby a little damping force is generated.
Such a conventional hydraulic shock absorber as described above has the following problems.
Since the hydraulic shock absorber requires a free piston 9 or a diaphragm so that the volume of the oil chamber 7 changes according to internal pressure, machining of the hydraulic shock absorber demands precise dimensional tolerances. In addition, the complicated structure causes high production costs.
Further, in order to secure the volume of the oil chamber 7, the oil chamber 7 should be arranged to extend long in the upper and lower directions (along the axial direction). This leads to a hydraulic shock absorber which is long in the total length and therefore difficult to miniaturize.
Moreover, either the extension stroke of the hydraulic shock absorber or the contraction stroke is only controlled. (The hydraulic shock absorber shown in FIG. 1 is adapted to control the extension stroke.) The identical two systems, disposed symmetrically with the upper and lower parts, would control the extension and contraction strokes. This is, however, impractical due to a hydraulic shock absorber extending lengthwise in the axial direction.